(1) Field of the Invention
The present invention relates to a solid-state image-taking system that includes: a solid-state image-taking apparatus for outputting electric charge read from light-sensitive elements; and a driving apparatus for driving the solid-state image-taking apparatus, and more specifically relates to a technology for preventing generation of a defective image such as a residual image, without increasing the read voltage.
(2) Description of the Related Art
In recent years, image-taking apparatuses such as home video cameras and digital still cameras have been in widespread use.
Among such image-taking apparatuses, there are ones that use solid-state image-taking devices of charge-transfer types that read electric charge from light sensitive elements and transfer and output the read electric charge.
FIG. 17 shows the structure of a portion of a conventional solid-state image-taking device of charge-transfer type, the portion corresponding to one unit pixel.
As shown in FIG. 17, in the solid-state image-taking device, a p-well 72 is formed on an n-type substrate 71. The p-well 72 includes an n-type area 73 being an embedded channel, and an n-type area 74 being a photodiode. Ap-type area 75 is provided between the n-type area 73 and the n-type area 74. A SiO2 film 76 is formed on the p-well 72. A gate electrode 77 made of polysilicon is formed above the n-type area 73 with the SiO2 film 76 in between. This structure of the conventional solid-state image-taking device of charge-transfer type is called MOS (metal-oxide semiconductor) structure.
The gate electrode 77 functions as a transfer electrode to transfer electric charge, and also functions as a read electrode to read electric charge from the photodiode.
Adjacent unit pixels are each separated by a p-type area 78 specifically provided for this purpose.
The p-well 72 is typically grounded. A reverse bias voltage is applied to the n-type substrate 71 for the p-well 72.
Here, varying the reverse bias voltage applied to the n-type substrate can change the potential depth of the photodiode, and can change the height of the potential barrier between the n-type substrate 71 and the photodiode. It should be noted here that what is called vertical overflow drain structure can be formed by setting the reverse bias voltage to such a value that allows electric charge excessively stored in the photodiode overflows to the side of the n-type substrate 71, not overflowing to the charge transfer channel. In the present document, overflowing of the electric charge excessively stored in the photodiode over to the charge transfer channel is referred to as “blooming”. Also, a voltage that is applied to the substrate and is at the limit of inhibiting the blooming is referred to as a “blooming inhibit voltage”. To prevent the blooming from occurring in the conventional solid-state image-taking device of charge-transfer type, it is necessary to form a potential barrier high enough to prevent the blooming, between the photodiode (n-type area 74) and the charge transfer channel (n-type area 73). It is therefore necessary to make the p-type impurity in the p-type area 75, which is provided for the potential control, dense enough to prevent the blooming, preventing the empty space between the photodiode and the charge transfer channel from broadening.
However, if the p-type impurity in the p-type area 75 is made dense enough to prevent the blooming and to prevent the empty space between the photodiode and the charge transfer channel from broadening, it becomes difficult for the empty space to broaden even if a positive voltage is applied to the gate electrode 77. When this happens, the read voltage must be set to a higher level to prevent signal charge from remaining.
Japanese Laid-Open Patent Application No. S62-145865 discloses a solid-state image-taking apparatus for preventing signal charge from remaining when the signal charge is transferred from the light-sensitive unit to the transfer unit, thus reducing an amount of residual image.
A defective image such as a residual image is generated when, for example, the signal charge remains because of the read voltage set to a low level.
On the other hand, there is a demand for decreasing the surface area of the unit pixel. However, if the surface area of the unit pixel is decreased, the horizontal width of the p-type area for the potential control is also decreased. This makes it necessary to increase the vertical depth of the p-type area. This also increases the read voltage. However, there is a limit to increasing the read voltage. As a result, sometimes the signal charge remains, and a defective image such as a residual image is generated.